1. Field of the Invention
The present invention relates to a urea-SCR system and a control method thereof.
2. Description of Related Art
Recently, owing to environmental pollution, related investigations have been actively undertaken so as to eliminate nitrogen oxide in exhaust gas, and the post-processing technology such as selective catalyst reduction (SCR) can be divided into two kinds.
First, there is a lean NOx trap (LNT) system in which a nitrogen oxide absorbing material is used. However, in order to use the above LNT system, a diesel engine is to be operated in a condition of less than a theoretical air/fuel ratio.
That is, in order to archive the lean air and rich fuel condition, a post-injection technique is to be used, that is, the fuel is further injected into an exhaust pipe such that the fuel consumption is increased.
Second, in order to eliminate the nitrogen oxide, there is a urea-SCR (urea-selective catalyst reduction system) that injects a urea solution into the exhaust pipe so as to eliminate the nitrogen oxide in the exhaust gas, and the injected urea is vaporized by thermal energy (at about 156° C.) of the exhaust gas or is decomposed by the catalyst to be transformed to ammonia, wherein the Reaction Formula 1 thereof is as follows.Reaction Formula 1(NH2)2CO+H2O→2NH3+CO2  Eq. (1)
The transformed ammonia reduces the nitrogen oxide in the exhaust pipe as in the following Reaction Formula 2 such that nitrogen and water are exhausted as the resultant materials thereof.Reaction Formula 24NH3+4NO+O2→4N2+6H2O  Eq. (2)4NH3+2NO2+O2→3N2+6H2O  Eq. (3)8NH3+6NO2→7N2+12H2O  Eq. (4)2NH3+NO+NO2→2N2+3H2O  Eq. (5)
The above technique that uses the ammonia to reduce the nitrogen oxide was developed about 20 years ago to be commercially available, and a detailed description thereof will be omitted.
Different from the final guidance program (distributed on Mar. 30, 2007) of the North American EPA for a vehicle, in which the urea-SCR is used, European laws stipulate that a reducing agent such as urea is to be monitored in a urea-SCR vehicle. Accordingly, in order to correspond to the above program, the control logic thereof needs to be investigated.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.